In contrast to Functional Magnetic Resonance Imaging (fMRI) methods that rely on the Blood Oxygenation Level Dependent (BOLD) effect to detect changes in brain perfusion, Spin Tagged Arterial Water Perfusion Magnetic Resonance Imaging (pMRI) yields quantitative data that in theory is directly comparable to perfusion imaging with Positron Emission Tomography (pPET). Unlike fully quantitated pPet, pMRI is non- invasive and does not use ionizing radiation. Nevertheless, pMRI has yet to be validated against an established perfusion imaging method like pPET. We propose a pilot study as a first step to validate pMRI in humans. Whole brain and regional cerebral blood flow (rCBF) measurements from the pMRI of normal volunteers will be compared against values from pPET, the current gold standard for in vivo measurement of brain perfusion. We aim to identify a correlation between these measurements, as well as, documents the intra and inter subject variation in the rCBF of normal subjects measured with pMRI. A quantitative description of these variations is necessary to document the reliabiity of the measurements and the range over which pMRI is valid.